Higgs boson on the horizon

Sep 5, 2000

'We all hope that what we are seeing is the Higgs on the horizon,' concluded Chris Tully of Princeton University to a packed CERN
auditorium this morning. Hundreds of particle physicists heard that all four experiments at the Large Electron Positron (LEP)
collider at the Geneva laboratory have found the first hints for the elusive Higgs boson at high energies. This particle - often
referred to as a 'holy grail' in particle physics - is thought to provide the mechanism through which particles have mass, and is
the last missing piece in the so-called standard model of particle physics. Slides from the talk are available as a pdf file on the Web.

Tully and co-workers have combined the data from the four experiments at LEP and found evidence that the Higgs boson has a mass of 114.9 GeV c-2. 'It is a 2.6 sigma effect,' he told PhysicsWeb, 'so there's still a 6 in 1000 chance that what we are seeing are background events, rather than the Higgs.'

The LEP collider was due to stop running at the end of September to make way for the Large Hadron Collider, a proton-proton collider that will be built in the same underground tunnel. Now there may be a strong scientific case for extending the lifetime of LEP until December to allow the experiments to double the amount of data collected. According to Tully, this will allow the teams to reduce the likelihood that the signal is some kind of background effect to 1 in 10 000.

Searches for the Higgs bosons have been underway at all four LEP experiments - ALEPH, DELPHI, L3 and OPAL - for several years. In recent months the energy of the electron and positron beams has been raised slowly and the accelerator is now running at its maximum possible centre-of-mass energy of 207 GeV.

The strongest evidence for the Higgs boson comes from the ALEPH experiment. Three 'events' were found which suggest that a Higgs boson was produced along with a Z boson, a neutral particle that carries the electroweak force. In these events, both the Higgs and the Z boson decay quickly to form quarks that cause a spray of particles in the detector. While the other three experiments find less compelling evidence, they cannot rule out the ALEPH data.

The international research board at CERN will decide on 14 September whether the LEP accelerator will continue running until December. Particle physicists await with bated breath.